South African modern beach sand and plate tectonics

Abstract

South America is the ideal continent for baseline studies on how the framework composition of modern sands relates to reconstruction of palaeoplates—it has well-defined active and passive margins, great contrasts in climate and relief and is free of continental glaciation (Fig. 1). I report here petrographical studies of its beach sands which show two active margin associations rich in volcanics and plagioclase—one along its Pacific coast and one along the Argentine coast—and two associations rich in quartz—a passive margin association from Argentina to Trinidad and a moderately, quartz-rich association along the Caribbean coast. Tectonic control is thus overwhelming but complications exist along the Argentine coast where a dry, narrow continent permits active margin sand to mantle a passive margin. Another surprising result is that quartz-rich sands border the crystalline rocks of much of the Sera do Mar in Brazil.